1. Field of the Invention
The present invention relates to a code converter wherein a digital code encoded into any desired bit length at any desired sampling frequency is converted into a digital code encoded into a shorter bit length at the same sampling frequency.
2. Description of the Related Art
In general, in a case where a digital code of n bits is rounded into a digital code of m bits (m&lt;n), S/N degrades 6.0.times.(n-m) dB. The degradation, however, can be improved by employing an oversampling technique. The oversampling technique is a technique wherein a high S/N characteristic and a good gain tracking characteristic are attained with a short bit length by encoding at a high sampling frequency as compared with a signal frequency.
A conventional code converter employing the oversampling technique is shown in FIG. 1. The digital integrator 1 in FIG. 1 is configured of an adder 5 and a one-sample delay unit 6 as shown in FIG. 2. The digital comparator 2 is realized by rounding off lower bits.
In the shown code converter, an input signal applied to the input terminal IN is subtracted by a subtracter 4 with an output signal produced at an output terminal OUT and delayed for one sampling period by a delay unit 3, to obtain a difference signal. The difference signal is integrated by a digital integrator 1. The integrated output is rounded into a bit length shorter than that of the input signal by a digital comparator 2.
Letting Q(t) denotes quantization noise developed by the code converter, and I(t) and 0(t) denote the input signal and the output signal,, respectively, a relational formula is found and then is Z-transformed, thereby to obtain the following equation: EQU O(Z)=I(Z)+(1-Z.sup.-1) Q(Z) (1)
As apparent from Eq. (1), the quantization noise is differentiated and is then transferred to the output, so that quantization noise components within a band become very small.
In the prior-art code converter stated above, the comparatively upper bits of the output code do not change from those of the input code, so that the operations of the digital integrator 1, the digital comparator 2 and the one-sample delay unit 3 become wasteful. This leads to enlarging a hardware quantity or requiring a large number of circuit elements, which is unsuitable for an integrated circuit.
Further, in general, the quantization noise is not white noise, but it correlates to the frequency or amplitude level of the input signal, and there arises a frequency or level at which the S/N characteristic or gain tracking characteristic degrades abruptly. As an expedient which is often adopted for improving such a phenomenon, there is well known a method in which dithers are applied to the input of the digital comparator 2. However, a circuit for generating the dithers is required, and the optimum spectrum of the dithers need to be selected depending upon the input signal. Accordingly, increase in hardware quantity or in the number of circuit elements and complication in design are incurred.
Besides, when the digital code is expressed by a sign magnitude form, the gain tracking characteristic (linearity) degrades due to round-off.